The Royals recalled Castillo from Triple-A Omaha ahead of Sunday's game against the Athletics.
Castillo will provide the Royals with a fresh arm capable of covering multiple innings out of the bullpen after Kansas City needed six relievers to cover 8.2 frames between the first two games of the series with Oakland. The Royals still plan on developing Castillo as a starter, so he'll likely be in line for only a brief stay with Kansas City before returning to Triple-A and slotting back into the Omaha rotation.